This invention relates to a method for the injection molding of a blind rivet comprising a shank and a rivet body formed around the shank so that an upward slide of the shank relative to the rivet body causes the rivet body to expand outwardly and the outward expansion of the rivet body enables the rivet to be fastened to a given panel by causing the panel to be squeezed between the expanded portion of the rivet body and a radially projecting head of the rivet body.
Ordinary blind rivets comprise a shank having a radially projecting lower end and a rivet body formed to enclose the shank tightly therein. To join two panels to each other, for example, this blind rivet is inserted in registered holes perforated in advance in the two panels, with the lower end of the rivet body in the lead and, then, the shank is slid up the rivet body so that its radially projecting lower end causes the rivet body to spread out radially. Consequently, the radially spread portion of the rivet body presses the two panels against the radially projecting head portion of the rivet body so fast that the two panels are brought into tight mutual adhesion. After the panels have been joined, it is usual practice to cut off the portion of the shank which protrudes from the rivet body and will no longer serve its purpose.
It is obvious that the force with which the blind rivet holds the panels in tight adhesion can be increased by simply increasing the extent to which the portion of the rivet body protruding past the panels is spread out radially by the upward slide of the shank.
Such an increase in the extent of the radial spreading of the rivet body can be realized by simply increasing the diameter of the radially projecting lower end of the shank. If the diameter is excessively increased, however, there ensues a possibility that the rivet body surrounding the shank will sustain cracks or the shank itself will break.
An excellent blind rivet which is capable of holding the panels with enhanced force and, further, sealing the holes perforated in the panels with perfect water-tightness heretofore unattainable by any existing blind rivets was previously developed and put to actual use by the inventor. The salient feature of this blind rivet resides in the rivet body being so adapted that the upward slide of the shank relative to the rivet body will cause the rivet body to be bent and folded over itself to increase the diameter thereof consequently, in contrast to the principle of the conventional blind rivet wherein the rivet body is spread out radially by the upward slide of the radially projecting lower end of the shank. Consequently, the inventor's blind rivet enjoys a greater increase in the diameter of the rivet body and permits the panels to be held in fast adhesion with enhanced force and the holes in the panels to be sealed with perfect water-tightness. Here again, due precaution must be exercised so that the rivet body made of a synthetic resin should not sustain cracks when it is deformed and finally folded over itself by the upward slide of the shank. An attempt to preclude the occurrence of such cracks solely by giving ample thickness to the wall of the rivet body is not desirable. When a synthetic resin is injection molded to produce a shaped article, the molten resin does not uniformly flow into the cavity of the die and, therefore, solidifies non-uniformly. In the injection molding of a hollow shaped article, for example, when the molten resin is injected through one injection orifice, the flow of the molten resin is divided by the core in the die which corresponds to the hollow space in the shaped article into two streams, which then wrap around the core corresponding to the shank placed in the second die used for molding the rivet body and eventually join again in one plane within the die. The plane wherein the streams of the molten resin join as described above is generally called a "weld line", which forms the most vulnerable part of the shaped resin article. In the case of a blind rivet to be produced by this technique, if the wall of the rivet body is thin, then the rivet body tends to sustain cracks along this weld line when the shank is slid up the rivet body. To prevent the occurrence of such cracks along the weld line, it has been usual practice to increase the wall thickness of the rivet body or minimize the extent of the radial spreading of the rivet body due to the upward slide of the shank. Either of the measures involves an unwanted sacrifice of operational ease or gripping force of the rivet. The method described above, therefore, is hardly suitable for the manufacture of the blind rivet previously developed by the inventor wherein the rivet body is bent out of shape and eventually folded over itself by the upward slide on the shank.
An object of the present invention is to provide a method for the injection molding of a plastic blind rivet consisting essentially of a shank and a rivet body such that the upward slide of the shank relative to the rivet body causes the rivet body to be bent and eventually folded over itself and the consequent increase in the wall thickness of the rivet body enables the rivet to be fastened to a given panel and the hole perforated in the panel to be sealed with perfect water-tightness, which method comprises preventing the produced plastic blind rivet from sustaining structural flaws such as, for example, uneven resin distribution and occurrence of weld line which degrade the product quality.